Find: 2 Ethers, 1 Potion, & 1 Tent

Board the train and when the I.D. Check fails, follow everyone else to get off the cars before the time limit expires. Eventually you are forced to jump off the train and end up on the tracks.. If you run down five screens, you can fight Shinra Security Guards (Special Combatants), and build some experience. Otherwise, go up a screen, to the green security sensors. Walk left, up to the small hatch, and climb in.
Walk forward and there is a green Ether on the ground. Climb down the ladder to the grate, and then down another ladder. Run left and talk to Wedge, then go up either ladder. Talk to Jessie, then look just to the left of the vent with the red triangle pointing at it. A Potion is hiding around the corner! Get it, and then proceed left and down the ladder. Run around to the little white flashing bag, which is a Tent. Now go to the Save Point to save. Then talk to Biggs, and go up the ladder.
Slide down the pipe at the right end of the ledge. You should recognize the layout of the reactor. Run left to the door at the base of the stairs. Follow the same path as you did in the Number 1 Reactor. When you reach the core, Cloud has a flashback of Tifa going into a reactor. Sabotage the computer, and leave the reactor core. Exit the same way you did the other reactor, up the pipes, up the long staircase, and use the elevator.
Upon exiting the elevator, open the chest on the left for another Ether. Go up and left into the room with the large panel in the center. Walk up to the panel, and time when you hit the button to match Tifa and Barret's hits. Now go through the open door and save. This may be a good time to use your Tent, because through the next door and down the stairs is the boss. When you reach the T-junction, talk to President Shinra. He flies away, but leaves you to fight Air Buster.